Traditionally, an induction generator can be started up as a motor using grid power to establish an excitation. With this method, the machine starts as a motor (i.e., consuming power), and it runs at slightly less than the grid synchronous speed. In an ORC machine, refrigerant vapor is subsequently directed to the turbine inlet, and is expanded by the turbine. As the flow of refrigerant increases, the induction machine changes its operating mode from a motor to a generator. As a generator, it runs at a speed slightly higher than the synchronous speed.
For large induction machines, starting as a motor requires the use of additional protection measures to avoid high inrush current. Low-voltage starters such as an autotransformer or a Wye-Delta starter are commonly used.
U.S. Pat. No. 5,714,862, issued to Mikesell et al. on Feb. 3, 1998, discloses a rotational speed measurement technique for a coasting induction motor. The speed is measured by sensing the residual electrical voltages at the power terminals of the motor, thus eliminating the need for conventional tachometer equipment, additional mechanical components, or modifications to the induction motor itself.